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Fw: Foundation Design in Seismic Zones

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This is a matter which has piqued my curiousity in the past as well.

It is only logical that the structural foundation system should have the
capacity to resist the loads imposed upon it by the superstructure either
using strength or ductility.  In other words, the foundation system should
be treated no differently from the superstructure.

I have asked a number of other engineers this question, and the general
consensus seems to be that such an approach is overly conservative -  a
view which I do not hold.  Most of the support given for this view (to me,
anyway) has to do with the inherent overstrength of the foundation system,
and the fact that the underlying soil will give some during a seismic

As far as the UBC goes, section 1921.2.2.3 states "Structural members below
base of structure required to transmit to the foundation forces resulting
from earthquake effects shall also comply with the requirements of Section

First question would be, what is the "foundation".  Are they talking about
the structural foundation system, or the underlying soil/rock?  I take the
view that it is the actual soil/rock.  Proceeding with this, the UBC would
then requrie that the foundation be designed in one of two ways:

1)  Using code level reduced forces, but designed for ductile performance.
2) Designed for a higher level of force (such as 3Rw/8), with minimal
ductile detailing.

I personally tend to shy away from ductile foundations, primarily because
it would be difficult if not impossible to determine the extent of damage
and repair it after a major earthquake.

T. Eric Gillham PE

> From: Donald W. Ross <don_ross(--nospam--at)>
> To: 'seaoc(--nospam--at)'
> Subject: Foundation Design in Seismic Zones
> Date: Sunday, May 17, 1998 2:18 PM
> The National Building Code of Canada contains a specific provision that
foundations in seismic zones be designed for the "nominal" capacity of the
structure.  I have found no similar provision in the UBC.
> Does such a provision exist in the UBC?  If a structure is designed using
a capacity based approach, then the foundation should be designed up to the
capacity of the structure.  This would ensure that the foundation is not
the weak link in the chain.
> Any thoughts?
> Best regards, Don Ross